Ferroelectric materials have many important applications as functional materials in electronics and optics. A large number of ferroelectric ceramics exploit properties that are an indirect consequence of ferroelectricity, such as dielectric, piezoelectric, pyroelectric and electro-optic properties. With the development of ceramic processing and thin film technology, many new applications have emerged. The biggest uses of ferroelectric ceramics have been in areas such as dielectric ceramics for capacitor applications, ferroelectric thin films for non volatile memories, piezoelectric materials for medical ultrasound imaging and actuators, and electro-optic materials for data storage and displays.
The most commonly used piezoelectric material, lead zirconate titanate (PZT), has the chemical formula PbZrxTi1-xO3, and is a binary solid solution of PbZrO3 (orthorhombic) and PbTiO3 (tetragonal perovskite). PZT has a perovskite type structure with the Ti4+ and Zr4+ ions occupying the B site at random.
Other lead-based ferroelectric compounds include lead niobate, PbNb2O6, which has a tungsten bronze type structure. The tungsten bronze family has a more open structure compared with the perovskites and a wide range of cation and anion substitutions are possible without loss of ferroelectricity. However, it is difficult to fabricate piezoelectric PbNb2O6-type ceramics because of the formation of a stable non-ferroelectric rhombohedral phase on cooling to room temperature. This problem has been addressed by rapid cooling from the sintering temperature, however, there is a further problem associated with this type of material, the large volume change due to phase transformation on cooling below the Curie point, leading to cracking of the ceramic.
Lead bismuth niobate (PbBi2Nb2O9) has a bismuth oxide layered type structure consisting of corner-linked perovskite-like sheets separated by (Bi2O2)2+ layers. The plate like crystal structure of these compounds leads to highly anisotropic ferroelectric properties. However, the piezoelectric properties are not good because of a very low poling efficiency. Poling is a process used to induce piezoelectric behaviour in a ferroelectric ceramic by applying a direct current electric field at a high temperature that is below the Curie point of the material. On application of the external field the spontaneous polarisation within each domain of the ceramic is orientated in the direction of the applied field, leading to a net polarisation in the poling direction, but the domains in a ceramic cannot fully align along the poling axis because the orientation of the polarisation is restricted by the symmetry of the crystal structure. For example, an orthorhombic perovskite has polarisation oriented along one of the eight [111] directions. The higher the number of possible orientations, the better the poling efficiency. Hence, compounds with a higher symmetry would be expected to have better poling efficiency and better piezoelectric properties, for example, a tetragonal or orthorhombic structure would be expected to have better poling efficiency than a monoclinic structure.
It has been possible to improve the piezoelectric properties of bismuth oxide layered type structures by grain orientation during the processing step, for example by tape casting or hot forging of the ceramic. The bismuth oxide layered structured ferroelectrics may become important piezoelectric ceramics because of their higher stability, higher operating temperature (Curie temperatures in the range of 550-650° C.) and higher operating frequency. These ceramics are mainly useful for piezoelectric resonators which need to exhibit a very stable resonant frequency.
In J Am Ceram Soc 91, [8], 2766-2768, Pb(Mg1/3Nb2/3)O3 is disclosed as a candidate for capacitor dielectrics and electroconstrictive actuators. The compound is made by spray pyrolysis.
There has been increased interest recently in lead-free piezoelectric/ferroelectric materials because the most commonly used piezoelectric ceramics are lead-based and are being phased out for environmental reasons relating to the toxicity of electronic waste.
The present inventors have realised that the most attractive materials to replace PZT are the alkali metal niobates. K0.5Na0.5NbO3 (KNN), LixNa1-xNbO3 wherein x<0.2 and KNN—LiNbO3, composite materials and doped derivatives thereof, have been described, for example by Sun et al. in Sci. Technol. Adv. Mater., Vol. 9, 2008, pp. 1-4, “BiFeO3-doped (Na0.5K0.5)NbO3 lead-free piezoelectric ceramics”. Also, in US2008/01355798 lead free piezoceramic K0.5Na0.5NbO3 compounds are described but these compounds are formed by solid state chemistry. In Materials Chemistry and Physics 77(2002) 571-577, thin films of LiNbO3 are deposited on silicon substrates by sputtering or spray pyrolysis.
Ferroelectric crystals grown from solid solutions of alkali and alkaline earth niobates have shown potential for use in laser modulation, pyroelectric detectors, hydrophones and ultrasonic applications. However, the traditional synthesis route for ceramic powders by firing oxides of the different cations gives a coarse powder which needs a very high sintering temperature. This is a particular problem here due to the volatilization of the alkali metals above about 1000° C., and poor sinterability has been the main obstacle for the development of these materials. The main challenge is the provision of single phase, fine-grained and dense ceramics, preferably with a high degree of preferential orientation in order to obtain improved ferroelectric properties. A high quality ceramic powder is defined by having a small particle size, narrow particle size distribution, low degree of agglomeration, high purity and high phase purity; hence sub-micron powders of these components are attractive possibilities in the development of lead-free piezoelectric and ferroelectric materials.
As noted in US2008/01355798, the use of solid state reaction method results in particle sizes greater than hundreds of nanometers. In order to manufacture particles of an acceptable size, the inventors in US2008/01355798 mill the particles. When a powder is made by a solid state reaction method a high temperature is necessary for the different oxides to react with each other. At this high temperature, coarsening of the particles occurs and they become large. The particles therefore must be milled to reduce the size of the particles, also known as primary particles or crystallites, into smaller particles. To break down these primary particles requires a lot of energy and breaking them down to sub micron level, for example less than 500 nm, is problematic. The present inventors have found a process which makes the necessary small particles or crystallites directly.
The synthesis route of the present invention starts with a stable solution of cations to build up the material. To achieve a small particle size from the aqueous solution, spray pyrolysis is used to directly prepare the oxide powder. By using spray pyrolysis, a large scale preparation of high quality powder is possible. The process of the present invention can be applied to form alkali metal niobates or mixed niobium-tantalum oxide solid solutions. It is also possible to dope the compositions with small amounts of other elements by adding salts, e.g. nitrate(s), of the dopant(s) to the solution.